1. Technical Field
The present invention relates to head up displays (HUD) and more particularly to head or helmet mounted HUD systems.
2. Discussion
HUD systems are commonly used in aircraft or other vehicles to display information, such as images of instruments in the field of view of a pilot or operator. Examples of such systems are described in U.S. Pat. Nos. 3,737,212 and 4,218,111. In U.S. Pat. No. 4,613,200 a HUD system employs holographic elements to project an image of an instrument into the viewer's field of view as the viewer looks through the forward window of a vehicle. In HUD systems the display apparatus may either be attached to the aircraft or vehicle or to the user's head or helmet.
Also related to the present invention are night vision goggle systems. These systems permit vision under very low light conditions by intensifying incoming infrared and/or visible light from a viewed object and converting it to intensified visible light. Night vision goggles transmit an electronically processed image into the viewer's field of view. Night vision goggles are normally attached to the viewer's head or helmet. This is done because when the viewer moves his head it is desirable to have the viewing axis of the night vision system correspond to the viewing axis of the viewer's eye.
Night vision goggles and viewing systems are described in U.S. Pat. Nos. 4,563,061, 4,468,101 and 4,653,879. There are a number of problems with such systems. Typically they are heavy, cumbersome and unstable. The length and weight of such systems can make them unsuitable in applications where the viewer's head is subjected to high gravitational or centrifugal loads, such as in high performance aircraft.
Further, because such systems often protrude a considerable distance in front of the viewer's eye they can present a clearance problem during an emergency pilot ejection. Another disadvantage is that the optical components in most prior systems invert the image. This necessitates additional optical components to re-invert the image, and the result is added weight, an increase in length, and a forward shift of the system's center of gravity.
Another disadvantage with night vision systems is that they give the user a limited field of view, with little or no peripheral vision. This renders such systems essentially useless for applications requiring peripheral vision.
Finally, both HUD's and prior night vision systems frequently introduce aberrations into the image. These include coma, chromatic aberration, and astigmatism.
Accordingly, it would be desirable to provide a head or helmet mounted display system which is light, does not protrude significantly away from the head, has a relatively wide field of view and provides a non-inverted image with minimal aberrations.
Also, it would be desirable to provide a helmet mounted display system which can be used to display information from a cathode ray tube (CRT) or can be used to display the output of a night vision system, and which would enable the user to switch from one display to the other.